O2 saturation of myoglobin is measured by reflectance microspectrophotometry in single cells of dog gracilis muscles at rest and after phasic contraction. Functional capillary density is measured in heart and skeletal muscles by direct microscopy under various conditions of O2 consumption and supply, and mean flow velocity and mean transit time in capillaries are calculated. Probability models are applied to estimate the frequency distribution of transit times. From the foregoing data, O2 gradients in capillaries and tissue are calculated. Results are compared with O2 gradients measured directly by myoglobin spectroscopy.